As the interest in energy and environment was increased, the demand for a secondary battery has been rapidly increased. Particularly, research into a lithium secondary battery having high energy density and discharge voltage has been actively conducted.
Particularly, the lithium secondary battery has been mainly used as a power supply of a mobile phone, a notebook computer, a digital camera, and an electric vehicle.
However, even though the lithium secondary battery has excellent electric characteristics, the lithium secondary battery has low safety characteristics.
More specifically, in the case in which electrolysis reaction continuously occurs in the lithium secondary battery in an abnormal operation state such as over-charging, over-discharging, exposure to a high temperature, and an electrical short circuit, or the like, heat and gas may be generated therein. As a result, when the electrolysis reaction is further promoted at a high temperature and high pressure, ignition or explosion may be generated.
Particularly, this problem may cause a severe major accident in the case of a middle and large-sized battery pack in which a plurality of battery cells are provided.
In the middle and large sized battery pack, the plurality of battery cells or unit modules are provided in a frame having a predetermined space, and as the plurality of battery cells or unit modules are expanded, pressure in a case may be rapidly increased.
In order to solve this problem, the secondary battery module includes a fuse, a bimetal, and a battery management system (BMS). However, sufficient safety may not be secured through only these configurations.
Particularly, in the normal state, the BMS may sense an electrical fault (over-discharging, over-charging, over-current) and control all of the modules to secure the safety, but when the BMS is not operated in the abnormal state, it may be difficult to control all of the modules, and a risk that a plurality of battery cells are expanded to thereby be ignited or be exploded may be increased.
In order to solve this problem, Korean Patent Laid-Open No. 10-2004-0110535 entitled “Secondary Battery Assembly with Gas Discharge Mechanism by Case Rupture” has been suggested, which is shown in FIG. 1.
The secondary battery assembly 12 with a gas discharge mechanism by a case rupture is configured to include a case 11, a secondary battery 10 including an electrode assembly 24 received in the case 11, a secondary battery pack 15 in which the secondary battery 10 is embedded, and a protrusion shaped rupture mechanism 51 formed in the secondary battery pack 15.
A change in volume of the secondary battery in the abnormal operation state is shown by a dotted line in FIG. 1.
In the secondary battery assembly with a gas discharging mechanism by a case rupture shown in FIG. 1, when a unit cell is expanded by the abnormal operation, the pouch type battery case may be ruptured by the rupture mechanism, thereby making it possible to secure the safety.
However, the rupture mechanism is formed in a space in which the secondary battery is received, such that it is likely to damage the case at the time of assembling the unit cell, and in the case in which the secondary battery assembly is mounted and then moved or used in an environment in which vibration is frequently generated, the pouch type battery case may be damaged by the rupture mechanism even in the normal operation state.
That is, in the secondary battery assembly with a gas discharging mechanism by a case rupture, in an assembling process or in the normal operation state, durability of the secondary battery may be deteriorated.